Optical sheet, backlight unit, liquid crystal display device, and information device

ABSTRACT

When at least one scope of 0.5 mm square or more of a surface of an optical sheet having the unevenness is measured, height data of each of a plurality of pixels of an image obtained is determined, an approximated surface is calculated, from the height data of each pixel, for a minute region of 100 μm2 or less, and a calculation is repeatedly performed to obtain an inclination angle between: (i) a flat surface appearing after the unevenness is imaginarily removed and: (ii) the approximated surface, while two-dimensionally shifting the minute region at equal intervals along the flat surface by using at least one of the pixels as a unit to obtain a plurality of minute regions, a total area of some of the minute regions each having the inclination angle of 30° or more, accounts for 30% or more of a total area of all the minute regions.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/927,753, filed on Jul. 13, 2020, which application was itself acontinuation of International Application No. PCT/JP2019/035235, filedon Sep. 6, 2019, which international application further claims priorityto and the benefit of Japanese Patent Application No. 2018-215551, filedon Nov. 16, 2018. The entire disclosures of these applications arehereby incorporated by reference herein.

BACKGROUND Related Field

The present disclosure relates to an optical sheet, a backlight unit, aliquid crystal display device, and an information device.

Description of Related Art

In recent years, liquid crystal display devices (hereinafter, alsoreferred to as “liquid crystal displays”) are widely used as displaydevices in various information devices such as smartphones and tabletterminals. The mainstream backlights of liquid crystal displays includea direct type having a light source arranged on the back side of aliquid crystal panel and an edge-lit type having a light source arrangednear a side of a liquid crystal panel.

In cases of adopting a direct-type backlight, a distance from a lightsource to a diffusion member (diffusion plate, diffusion sheet,diffusion film) used for erasing a light source image is typically setlong, so as to erase the image of the light source such as LightEmitting Diodes (LEDs) or the like on the light emitting surface,thereby improving the uniformity of the in-plane luminance.

For this reason, while there are cases of adopting the direct backlightin a large liquid crystal display for use in, for example, a TV monitorand the like, the edge-lit backlight has been adopted in many cases insmall to medium size liquid crystal displays of, for example, a laptopPC, a tablet device, a car navigation system, a smartphone, and thelike, to meet the demand for a reduction in the thickness.

In recent years, for the purpose of improving the image quality on aliquid crystal display, development for supporting High Dynamic Range:widening the range of brightness (HDR) has been advanced. An approachcurrently discussed to support the HDR is to adjust the amount ofbacklight, in addition to a shutter function using liquid crystals, bycausing LEDs of the direct backlight to individually turn on, turn off,and perform light-amount adjustment (local-dimming).

There is also a demand for small to medium size liquid crystal displaysto support the HDR by the local-dimming method involving the directbacklight. However, the thicknesses of small and medium size liquidcrystal displays have been already reduced by adopting the edge-litmethod, and it is difficult to increase the thicknesses of such liquidcrystal displays for the purpose of supporting the HDR. For this reason,there is a need for reducing the thickness of the direct backlight.

An approach to improve the luminance uniformity of the light emittingsurface, while reducing the thickness of the direct backlight is toimprove the light diffusibility of the diffusion sheet or the like usedfor erasing the image of the light source. An example of doing so is toprovide a large quantity of diffusing agent to the diffusion sheet andthe like. In this regard, Japanese Unexamined Patent Publication No.2012-42783 proposes using an optical sheet having a plurality of lenseswith recesses. Further, Japanese Unexamined Patent Publication No.2008-103200 proposes an arrangement of LED light sources so that threeadjacent LED light sources constitute vertices of an equilateraltriangle.

BRIEF SUMMARY

However, even if a large amount of diffusing agent is provided in anoptical sheet (diffusion sheet) to improve the luminance uniformity ofthe light emitting surface of the direct backlight using a plurality oflight sources, non-uniformity of the luminance occurring between thelight sources and areas between the light sources (areas where no lightsources are arranged) cannot be sufficiently addressed (see paragraph[0009] of Japanese Unexamined Patent Publication No. 2012-42783).

As to the distance from the light source to an optical sheet or adiffusion plate for erasing the image of the light source, JapaneseUnexamined Patent Publication No. 2012-42783 discloses 20 mm (paragraph0048) and Japanese Unexamined Patent Publication No. 2008-103200disclose 25 mm (paragraph 0024). The distance, however, needs to be 15mm or less (preferably 10 mm or less, more preferably 5 mm or less,furthermore preferably 2 mm or less, and ultimately 0 mm), in order toachieve a reduction in the thickness of future small to medium sizeliquid crystal displays.

However, regarding a reduction in the thickness of future directbacklights, the technique of Japanese Unexamined Patent Publication No.2012-42783 to “arrange a plurality of lenses each having recesses on anoptical sheet” and the technique of Japanese Unexamined PatentPublication No. 2008-103200 to “arrange LED light sources so that threeadjacent LED light sources constitute vertices of an equilateraltriangle” may fall short for sufficiently improving the luminanceuniformity of the light emitting surface.

In view of the above, it is an object of the present disclosure tosufficiently suppress non-uniformity of the luminance between lightsources and areas between light sources on the light emitting surface,despite further progress in a reduction in the thickness of directbacklights for liquid crystal display devices.

To achieve the above object, the inventors have conducted a study onluminance uniformity on the light emitting surface of a directbacklight, using optical sheets of various surface shapes. As a resultof the study, the inventors have found that luminance uniformity islargely influenced by the light reflection characteristics of opticalsheets, in addition to the arrangement of the light sources and thelight diffusibilities of the optical sheets. As a result of assessingthe relationship between the luminance uniformity and various surfaceshapes of optical sheets, the inventors have arrived at the presentdisclosure as described below.

Namely, an optical sheet of the present disclosure is an optical sheetto be interposed between a plurality of small light sources and a prismsheet in a liquid crystal display device having the small light sourcesarranged in a dispersed manner at a side of the liquid crystal displaydevice opposite to a display screen. The optical sheet has at least onesurface with unevenness. When at least one scope of 0.5 mm square ormore of the surface of the optical sheet having the unevenness ismeasured, height data of each of a plurality of pixels of an imageobtained is determined, an approximated surface is calculated, from theheight data of each pixel, for a minute region of 100 μm² or lessincluding a plurality of pixels, and a calculation is repeatedlyperformed to obtain an inclination angle between: (i) a flat surface(with a height of 0) appearing after the unevenness is imaginarilyremoved and: (ii) the approximated surface, while two-dimensionallyshifting the minute region at equal intervals along the flat surface byusing at least one of the pixels as a unit to obtain a plurality ofminute regions, a total area of some of the minute regions each havingthe inclination angle of 30° or more, accounts for 30% or more of atotal area of all the minute regions, for which the calculation is made.

With the present disclosure, the unevenness is controlled so that thetotal area of the minute regions with surfaces inclined at inclinationangles of 30° or more accounts for 30% or more of the total area of allthe minute regions for which the inclination angles are obtained. Theinclination angles are obtained by approximating the surfaces of theminute regions with the unevenness to flat surfaces, while shifting theminute region with the predetermined projection area with respect to theimaginary plane of the optical sheet along the imaginary plane to obtainthe minute regions at respective points on the imaginary plane. Thiscontrol promotes the reflection of the light incident on the opticalsheet from the small light sources. Specifically, for example, multiplereflections in the optical sheet and multiple reflections between thereflection sheet, on which the light sources are placed, and the opticalsheet are promoted. As a result, even if the distance between the lightsources and the optical sheet decreases in the direct backlight providedwith the plurality of light sources such as LEDs, non-uniformity of theluminance between the small light sources and the areas between thelight sources on the light emitting surface sufficiently decreases.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a cross-sectional view of a liquid crystal display device ofan embodiment.

FIG. 2 is a cross-sectional view of a backlight unit of the embodiment.

FIG. 3 is a cross-sectional view of an optical sheet of the embodiment.

FIG. 4 includes two illustrations, the first being an illustration Athat illustrates an example of obtaining inclination angles of surfacesof minute regions in the optical sheet of the embodiment. Theinclination angles are obtained by approximating the surfaces of theminute regions with unevenness to flat surfaces, while a minute regionwith a predetermined projection area is shifted along the imaginaryplane. Illustration B of FIG. 4 illustrates another example of obtaininginclination angles of surfaces of minute regions in the optical sheet ofthe embodiment. The inclination angles are obtained by approximating thesurfaces of the minute regions with unevenness to flat surfaces, while aminute region with a predetermined projection area is shifted along theimaginary plane.

FIG. 5 illustrates that light incident from a light source is reflectedby an optical sheet of the embodiment.

FIG. 6 illustrates LED light sources viewed from above.

FIG. 7 illustrates LED light sources viewed from above through theoptical sheet of the embodiment.

FIG. 8 illustrates LED light sources viewed from above through atraditional optical sheet.

FIG. 9 illustrates a luminance distribution at a light source.

FIG. 10 illustrates a distribution in the x direction of the luminancedistribution shown in FIG. 9.

FIG. 11 is a picture of a surface shape of an optical sheet ofComparative Example 1

FIG. 12 is a picture of a surface shape of an optical sheet of Example 1

FIG. 13 is a picture of a surface shape of an optical sheet ofComparative Example 2

FIG. 14 is a picture of a surface shape of an optical sheet of Example 2

FIG. 15 is a picture of a surface shape of an optical sheet of Example 3

FIG. 16 is a picture of a surface shape of an optical sheet of Example 4

FIG. 17 is a picture of a surface shape of an optical sheet of Example 5

FIG. 18 illustrates values of the “roughness (Ra)”, the “ratio of thearea with inclination angles of 30° or more”, and the “FWHM” of opticalsheets of Examples 1 to 5 and those of Comparative Examples 1 and 2.

FIG. 19 illustrates a correlation between the “ratio of the area withinclination angles of 30° or more” and the “FWHM” of optical sheets ofExamples 1 to 5 and those of Comparative Examples 1 and 2.

FIG. 20 illustrates a relation between the “roughness (Ra)” and the“FWHM” of optical sheets of Examples 1 to 5 and those of ComparativeExamples 1 and 2.

FIG. 21 is a cross-sectional view of an optical sheet of anotherembodiment.

FIG. 22 illustrates values of the “content of diffusing agent”, the“film thickness”, the “surface shape and surface roughness”, the “FWHM”and the like of optical sheets of Examples 6 to 20.

FIG. 23 illustrates a cross-sectional structure of the backlight unit ata time of evaluating the luminance uniformity for the optical sheets ofExamples 21 to 32.

FIG. 24 illustrates an arrangement of light sources in the backlightunit shown in FIG. 23.

FIG. 25 illustrates values of the “content of diffusing agent”, the“film structure”, the “luminance uniformity”, and the like of opticalsheets of Examples 21 to 32.

FIG. 26 is a plan view of an optical sheet of a modification.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

The following describes, with reference to the drawings, an opticalsheet, a backlight unit, a liquid crystal display device, and aninformation device of an embodiment of the present disclosure. The scopeof the present disclosure is not limited to the following embodiment,and any given modification is possible within the scope of the technicalthought of the present disclosure.

FIG. 1 is an exemplary cross-sectional view of a liquid crystal displaydevice of the present embodiment. FIG. 2 is an exemplary cross-sectionalview of a backlight unit of the present embodiment. FIG. 3 is anexemplary cross-sectional view of an optical sheet of the presentembodiment.

As shown in FIG. 1, the liquid crystal display device 50 includes aliquid crystal display panel 5, a first polarizing plate 6 attached to aback surface of the liquid crystal display panel 5, a second polarizingplate 7 attached to a front surface of the liquid crystal display panel5, and a backlight unit 40 provided close to the back surface of theliquid crystal display panel 5 with the first polarizing plate 6interposed therebetween. The liquid crystal display panel 5 includes aTFT substrate 1 and a CF substrate 2 which are provided so as to faceeach other, a liquid crystal layer 3 which is provided between the TFTsubstrate 1 and the CF substrate 2, and a sealing material (notillustrated) which is provided in a frame shape so as to enclose theliquid crystal layer 3 between the TFT substrate 1 and the CF substrate2.

The shape of a display screen 50 a of the liquid crystal display device50 as viewed from the front (from above in the figure) is basically arectangular or square shape; however, the shape of the display screen 50a is not limited to this and may have a rectangular shape with roundedcorners, an elliptical shape, a circular shape, a trapezoidal shape, orany given shape of an instrument panel of an automobile, and the like.

In the above-described liquid crystal display device 50, a predeterminedvoltage is applied to the liquid crystal layer 3 in sub-pixelscorresponding to the respective pixel electrodes to change the alignmentstate of the liquid crystal layer 3, thereby adjusting the transmittanceof light incident from the backlight unit 40 through the firstpolarizing plate 6. Then, the light thus adjusted is emitted through thesecond polarizing plate 7, thereby displaying an image.

The liquid crystal display device 50 of the present embodiment is usedas a display device to be built in various information devices (e.g., anon-board device such as a car navigation system, a personal computer, amobile phone, a portable information terminal, a portable game machine,a copy machine, a ticket vending machines, an automatic teller machine,and the like).

The TFT substrate 1 includes, for example, a plurality of TFTs, aninterlayer insulating film, a plurality of pixel electrodes, and analignment film. The plurality of TFTs are arranged in a matrix on aglass substrate. The interlayer insulating film covers each of the TFTs.The plurality of pixel electrodes are arranged in a matrix on theinterlayer insulating film, and are connected to the respective TFTs.The alignment film covers each of the pixel electrodes. The CF substrate2 includes, for example, a lattice-like black matrix formed on a glasssubstrate, color filters including red layers, green layers, and bluelayers arranged in open regions of the lattice of the black matrix, acommon electrode covering the black matrix and the color filter, and analignment film covering the common electrode. The liquid crystal layer 3is made of, e.g., a nematic liquid crystal material including liquidcrystal molecules having electrooptic characteristics. Each of the firstpolarizing plate 6 and the second polarizing plate 7 includes, forexample, a polarizer layer having a polarization axis in one direction,and a pair of protective layers sandwiching the polarizer layer.

As shown in FIG. 2, the backlight unit 40 includes a reflection sheet41, a plurality of small light sources 42 two-dimensionally arranged onthe reflection sheet 41, an optical sheet 43 arranged above the smalllight sources 42, a first prism sheet 44 and a second prism sheet 45arranged in this order above the optical sheet 43, and a polarizer sheet46 arranged above the second prism sheet 45.

The reflection sheet 41 is configured as, e.g., a white film made of apolyethylene terephthalate resin, or a silver evaporated film.

The type of the small light source 42 is not limited. For example, anLED element, a laser element, or the like may be used. For the sake ofproductivity, an LED element may be adopted. Each of the small lightsources 42 may have a rectangular shape in plan view. In such a case,the length of one side may be 10 μm or longer (preferably, 50 μm orlonger) and 20 mm or shorter (preferably, 10 mm or shorter and morepreferably 5 mm or shorter). In cases of adopting an LED as the smalllight source 42, a plurality of LED chips of several millimeters squaremay be arranged on the reflection sheet 41 at regular intervals. A lensmay be provided to the LED to adjust the light emitting anglecharacteristics of the LED to serve as the small light source 42.

The optical sheet 43 has a diffusion layer 21, and an uneven layer 22formed on the diffusion layer 21, as shown in FIG. 3. For example, thediffusion layer 21 is made of polycarbonate as a base material, andcontains about 0.5 to 4% by mass of the diffusing agent 21 a withrespect to 100% by mass of the base material. The diffusing agent 21 amay be made of a known material as appropriate. For example, the unevenlayer 22 is made of clear polycarbonate. On the surface of the unevenlayer 22, for example, recesses 22 a each having an inverted pyramidshape are two-dimensionally arrayed. The vertex angles θ of the recesses22 a are 90°, for example. The recesses 22 a are arranged at pitch p ofabout 100 for example. The optical sheet 43 may have a single-layerstructure containing a light diffusing agent and having an unevensurface. As should be understood, the optical sheet 43 is not limited tothe one shown in FIG. 3. For example, the optical sheet may have asingle-layer structure with unevenness, or may be a multi-layer of threeor more layers including an uneven layer. The uneven layer is formed notonly by two-dimensionally arranging the recesses in the inverted pyramidshapes as described above, but also may be formed by randomly arrangingprojections and recesses.

The first prism sheet 44 and the second prism sheet 45 are, for example,each a film having a plurality of adjacent grooves having an isoscelestriangle-shaped cross-section, an adjacent pair of the grooves forming aprism whose vertex angle is about 90°. Here, the grooves formed in thefirst prism sheet 44 and the grooves formed in the second prism sheet 45are disposed so as to be orthogonal to each other. The first prism sheet44 and the second prism sheet 45 may be integrally formed. For example,the first prism sheet 44 and the second prism sheet 45 may be apolyethylene terephthalate (PET) film having prism shapes made of aUV-curable acrylic resin.

For example, the polarizer sheet 46 may be a DBEF series manufactured by3M. The polarizer sheet 46 improves the luminance of the display screen50 a, by blocking absorption of the light emitted from the backlightunit 40 into the first polarizing plate 6 of the liquid crystal displaydevice 50.

One feature of the present embodiment resides in the following point.Namely, at least one surface of the optical sheet 43 has an unevenness.In the optical sheet 43, a total area of some of minute regions, whichhave surfaces inclined at inclination angles of 30° or more from animaginary plane of the optical sheet 43, accounts for 30% or more of atotal area of all the minute regions, for which the inclination anglesare obtained by approximating the surfaces of the minute regions withthe unevenness to flat surfaces, while two-dimensionally shifting aminute region with a predetermined projection area with respect to theimaginary plane of the optical sheet 43 at equal intervals along theimaginary plane to obtain the minute regions at respective points on theimaginary plane.

Here, the imaginary plane of the optical sheet 43 means a flat surfacewhich remains after removing the unevenness of the optical sheet 43.More specifically, the “imaginary plane” denotes an imaginary planeprovided horizontally in contact with the deepest points of the recessesof the unevenness. However, the term “horizontally” here means being inparallel to the horizontal plane of the optical sheet 43, and does notmean the horizontal direction in a strict sense. For example, if thesurface of the optical sheet 43 opposite to the uneven surface is flator substantially flat, the imaginary plane may be provided in parallelto the opposite surface.

FIG. 4A illustrates an example of obtaining inclination angles (e.g.,θ1, θ2, θ3, . . . ) of surfaces (e.g., V1, V2, V3, . . . ) of minuteregions (e.g., R1, R2, R3, . . . ) from an imaginary plane (P) of theoptical sheet 43 (specifically, the uneven layer 22) in each opticalsheet 43. The inclination angles are obtained by approximating thesurfaces of the minute regions with unevenness (e.g., S1, S2, S3, . . .) to flat surfaces, while shifting a minute region with a predeterminedprojection area along the imaginary plane to obtain the minute regionsat respective points on the imaginary plane. In FIG. 4A, the minuteregion is shifted in accordance with the inclined surfaces of theunevenness for simplification. In practice, however, the minute regionis shifted two-dimensionally at equal intervals, regardless of the stateof the unevenness as shown in FIG. 4B. At this time, the minute regionat the point after the shift may overlap the minute region at the pointbefore the shift.

With the above-described embodiment, the unevenness is controlled sothat the total area of the minute regions with surfaces inclined atinclination angles of 30° or more accounts for 30% or more of the totalarea of all the minute regions, for which the inclination angles areobtained. The inclination angles are obtained by approximating thesurfaces of the minute regions with the unevenness to flat surfaces,while shifting the minute region with the predetermined projection areawith respect to the imaginary plane of the optical sheet 43 along theimaginary plane to obtain the minute regions at respective points on theimaginary plane. As shown in FIG. 5, this control promotes thereflection of the light incident on the optical sheet 43 from aplurality of small light sources 42. Specifically, for example, multiplereflections in the optical sheet 43 and multiple reflections between thereflection sheet 41, on which the light sources 42 are placed, and theoptical sheet 43 are promoted. As a result, even if the distance betweenthe small light sources 42 and the optical sheet 43 decreases in thedirect backlight unit 40 provided with the plurality of light sources 42such as LEDs, non-uniformity of the luminance between the small lightsources 42 and the areas between the light sources on the light emittingsurface sufficiently decreases.

In the present embodiment, the number of the small light sources 42 isnot limited. However, to be distributed, the light sources 42 may bearranged regularly on the reflection sheet 41 in one preferredembodiment. The phrase “arranged regularly” means that arranged with acertain regularity. Examples include the case where the small lightsources 42 are arranged at equal intervals. If the small light sources42 are arranged at equal intervals, the distance between the centers oftwo adjacent small light sources 42 may fall within a range from 0.5 mm(2 mm in one preferred embodiment) to 20 mm, both inclusive. A distanceof 0.5 mm or more between the centers of two adjacent small lightsources 42 tends to cause a phenomenon (non-uniformity of the luminance)that the region between the adjacent small light sources 42 has a lowerluminance than the other regions. This makes the present embodiment moreuseful.

FIG. 6 illustrates LED light sources viewed from above. FIG. 7illustrates LED light sources viewed from above through the opticalsheet 43 of the present embodiment. FIG. 8 illustrates LED light sourcesviewed from above through a traditional optical sheet (the ratio of thetotal area with surfaces inclined at inclination angles of 30° or moreis less than 30%).

While the optical sheet 43 of the present embodiment decreases thenon-uniformity of the luminance between the light sources and the spacebetween the light sources as shown in FIG. 7, there is non-uniformity ofthe luminance between the light sources and the space between the lightsources with the traditional optical sheet as shown in FIG. 8.

The shapes of the unevenness of the surface of optical sheet 43 are notuniform and may be different to some extent (see FIG. 4A), since theprocessing accuracy is limited in industrial production. In this case,the inclination angles of the surfaces (i.e., the approximated surfaces)of the minute regions on the uneven surface of the optical sheet 43 aredifferent within a range from 0° to 90°. In this embodiment, theunevenness is controlled so that 30 percent of the surfaces haveinclination angles of 30° or more when the differences are aggregated.In particular, an optical sheet 43 made of plastic film has difficultyin uniformizing the projections or recesses on the surfaces of the lightdiffusers 43. It is thus largely advantageous to apply this embodiment,which assumes that the inclination angles of the surfaces of the minuteregions are different.

While the upper surface (i.e., the surface closer to the first prismsheet 44) of the optical sheet 43 is uneven (i.e., has recesses 22 a) inthis embodiment, at least one of the surfaces of the optical sheet 43may be uneven. That is, the lower surface (i.e., the surface closer tothe small light source 42) or both the surfaces (i.e., the upper andlower surfaces) of the optical sheet 43 may be uneven.

The unevenness of the surfaces of the optical sheet 43 is not limited,as long as the inclination angles of the surfaces of the minute regionsare measurable by, for example, a method described later. The unevensurface may be, for example, a matte surface with a random pitch,arrangement, or shape. Alternatively, a plurality of projections andrecesses may be arranged two-dimensionally.

The unevenness of the surfaces of the optical sheet 43 may includepolygonal pyramids or shapes that can be approximated to polygonalpyramids. Here, the term “polygonal pyramid” means a triangular pyramid,quadrangular pyramid or hexagonal pyramid which can be arranged tightlyon the surfaces of the optical sheet 43 in one preferred embodiment. Thetight arrangement of the polygonal pyramids and the shapes that can beapproximated to the polygonal pyramids on the surface of the opticalsheet 43 reduces the total area of the regions inclined at inclinationangles of 0° from the imaginary plane of the optical sheet 43. Inaddition, the surface of the optical sheet 43 is formed uneven by amanufacturing process such as extrusion molding or injection moldingusing a die (e.g., metal rolls). In view of the accuracy in cutting thesurface of the die (or each metal roll), the polygonal pyramids may bequadrangular pyramids.

Examples of the projections may include hemispheres (i.e., upperhalves), cones, triangular pyramids, quadrangular pyramids, andhexagonal pyramids. Examples of the recesses may include hemispheres(i.e., lower halves), inverted cones, inverted triangular pyramids,inverted quadrangular pyramids, and inverted hexagonal pyramids.

Examples of the projections may further include substantial hemispheres(i.e., upper halves), substantial cones, substantially triangularpyramids, substantially quadrangular pyramids, and substantiallyhexagonal pyramids. Examples of the recesses may further includesubstantial hemispheres (i.e., lower halves), substantially invertedcones, substantially inverted triangular pyramids, substantiallyinverted quadrangular pyramids, and substantially inverted hexagonalpyramids. Here, the phrase “substantial(ly) XX” means that shapes can beapproximated to the XX. For example, the phrase “substantiallyquadrangular pyramids” means that shapes can be approximated to thequadrangular pyramids. In fact, the projections and recesses may bedeformed from substantial hemispheres (i.e., upper and lower halves),substantial cones (or substantially inverted cones), substantially(inverted) triangular pyramids, or substantially (inverted) quadrangularpyramids in view of the accuracy in industrial production. There may beinevitable variations in the shapes caused by the processing accuracy ofindustrial production.

If a plurality of projections and recesses are arrangedtwo-dimensionally on the surface of the optical sheet 43, theprojections and recesses may be arranged tightly over the entiresurfaces of the optical sheet 43. Alternatively, the projections andrecesses may be arranged at regular intervals (i.e., a constant pitch)or irregular intervals.

In this embodiment, as long as the inclination angles of the unevensurfaces of the optical sheet 43 can be obtained for each minute region,the method of calculating the inclination angles is not limited. Forexample, the following method may be employed.

Step 1: With the use of a laser microscope VK-100 manufactured byKeyence Corporation, the surface shapes of the optical sheet 43 aremeasured at a magnification of 400. Automatic inclination correction isthen performed to collect the height data of 1024 pixelshorizontally×768 pixels vertically (or 697 μm×522.6 μm) as acomma-separated values (CSV) file. For example, the height may bemeasured as follows. First, after the point of focus is changed stepwiseto obtain a plurality of confocal images to be measured, a variationcurve of light intensity (I-Z curve) is estimated for each pixel basedon the relationship between the discrete points of focus (Z) and thelight detection intensity (I). From the I-Z curve, the peak position,that is, the height is obtained.

Step 2: The height data (digits) collected in Step 1 is converted intoheight data (μm) using a Z-calibration value.

Step 3: The height data obtained in the Step 2 is used to calculate aplane, to which the surface of a sheet in a minute region of 4×4 pixels(with an area (i.e., the projection area with respect to the imaginaryplane of the optical sheet 43) of 7.29 μm²) can be approximated, by aknown mathematical method based on the data of the minute region.

Step 4: An angle between the approximated surface calculated in Step 3with the imaginary plane of the optical sheet 43 (i.e., a plane having aheight of 0) is calculated to serve the inclination angle of the minuteregion.

Step 5: Steps 3 and 4 are performed for minute regions at 500,000 ormore points (specifically, 779,280 points), while horizontally orvertically shifting the minute region one pixel by one pixel.

Step 6: The percentage of the total area of the minute regions withinclination angles of 30° or more (hereinafter referred to simply as a“ratio of the area with inclination angles of 30° or more”) to the totalarea of the minute regions at the 500,000 or more points measured inStep 5 is calculated.

In one preferred embodiment, the “ratio of the area with inclinationangles of 30° or more” is calculated from the height data of the scope(in the range of 522.6 μm×697 μm in Step 1) of 0.5 mm square or more inview of the processing variations as described in Step 1, for example.As a matter of course, the height data may be obtained for a pluralityof scopes of 0.5 mm square or more, or for the entire surface of thesheet as a scope to improve the accuracy of the data. With an increasein the scope, the number of the “minute regions” at which theinclination angles are calculated also increases. There is no particularupper limit to the number of the “minute regions,” as long as they haveno problem as resources for, for example, measurement or dataprocessing. For example, if two scopes of 0.5 mm square or more are set,the number of “minute regions” also doubles. Also, even in the scope ofthe same size, the number of the “minute regions” increases with anincrease in the number of pixels included in the scope. That is, thenumber of “minute regions” whose inclination angles are to be calculateddepends on, for example, the size of the scope, the number of pixelsincluded in the scope, and the areas of the minute regions; which willbe described later. However, if the scope has an area of 0.5 mm squareor more in view of processing variations, for example, the inclinationangles of at least 100,000 or more (preferably 300,000 or more, and morepreferably 500,000 or more) minute regions may be calculated toaccurately figure out the unevenness.

In the above-described method described above, the inclination angle wascalculated, while horizontally or vertically shifting the minute regionof 4×4 pixels on a one-pixel basis. Instead, the inclination angle maybe calculated, while horizontally or vertically shifting a minute regionof the same size on a two-pixel basis. Alternatively, the inclinationangle may be calculated, while horizontally or vertically shifting theminute region of a larger size (e.g., 8×8 pixels) on a four-pixel basis.In short, the pixel sizes of the minute regions and the pitch ofshifting the minute region can be set freely, as long as unevenness isaccurately figured out.

The area (i.e., the projection area respect to the imaginary plane ofthe optical sheet) of the minute region in measuring the inclinationangle is not limited, as long as being sufficiently small to evaluatethe reflection characteristics of light from a small light source at themicro level. In view of the measurement accuracy, the performance ofmeasurement equipment or the like, the area may be 0.1 mm² or less(preferably 0.01 mm² or less, more preferably 0.001 mm² or less, andfurther preferably 0.0001 mm² (i.e., 100 μm²) or less, 7.29 μm² in Step3).

In this embodiment, the optical sheet 43 including the diffusion layer21 with the diffusing agent 21 a promotes light diffusion in the opticalsheet 43, thereby further reducing the non-uniformity of the luminancebetween the small light sources 42 and the areas between the lightsources.

The material of the diffusing agent 21 a contained in the diffusionlayer 21 is not limited. Examples may include silica, titanium oxide,aluminum hydroxide, and barium sulfate as inorganic particles, as wellas acryl, acrylonitrile, silicone, polystyrene, and polyamide as organicparticles.

The particle size of the diffusing agent 21 a may fall within a rangefrom 0.1 μm (preferably 1 μm) to 10 μm (preferably 8 μm), bothinclusive, for example, in view of the light diffusing effect.

The concentration of the diffusing agent 21 a may fall within a rangefrom 0.1% (preferably 0.3%) by mass to 10% (preferably 8%) by mass, bothinclusive, with respect to 100% by mass of the material (i.e., thematrix) of the diffusion layer 21, for example, in view of the lightdiffusing effect.

The difference in the refractive index between the diffusing agent 21 aand the matrix of the diffusion layer 21 may be 0.01 or more, preferably0.03 or more, more preferably 0.05 or more, further more preferably 0.1or more, and most preferably 0.15 or more. A difference of less than0.01 between refractive index of the diffusing agent 21 a and therefractive index of the matrix of the diffusion layer 21 causesinsufficient diffusion effects of the diffusing agent 21 a.

The resin to be the matrix of the diffusion layer 21 is not limited, aslong as being a material that transmits light. Examples may includeacryl, polystyrene, polycarbonate, methyl methacrylate-styrene copolymer(an MS resin), polyethylene terephthalate, polyethylene naphthalate,cellulose acetate, and polyimide.

The thickness of the optical sheet 43 of this embodiment is not limited,but may fall, for example, within a range from 0.1 mm to 3 mm(preferably to 2 mm, more preferably to 1.5 mm, and further morepreferably to 1 mm), both inclusive. The optical sheet 43 with athickness larger than 3 mm makes it difficult to achieve a reduction inthe thickness of the liquid crystal display. On the other hand, theoptical sheet 43 with a thickness smaller than 0.1 mm makes it difficultto achieve the effect of improving the luminance uniformity, which hasbeen described above.

If an optical sheet has a multilayer structure (e.g., the diffusionlayer 21 as the lower layer and the uneven layer 22 as the upper layer)like the optical sheet 43 of this embodiment, a layer (i.e., the unevenlayer 22) with an uneven surface has a thickness that is greater thanthe maximum height or depth of the unevenness. For example, a layer withprojections (or recesses) with height (or depth) of 20 μm has athickness larger than 20 μm.

In this specification, the term “optical sheet” means a sheet having anoptical function such as diffusion, collection, refraction, andreflection. As described above, the optical sheet 43 of this embodimentincludes, on the diffusion layer 21, the uneven layer 22 with the unevensurface of the present disclosure. The optical sheet 43 may be replacedwith a single-layer optical sheet 43 containing a diffusing agent andhaving an uneven surface. Alternatively, the optical sheet 43 maycontain three or more layers including the diffusion layer 21 and theuneven layer 22. Alternatively, the diffusion layer 21 and the unevenlayer 22 may be independent optical sheets, which may be layered orindependently arranged. In the latter case, the uneven layer 22 may bedisposed closer to the small light sources 42. Alternatively, theoptical sheet 43 may only include the diffusion layer 21 and the lowersurface of the first prism sheet 44 may have the unevenness of thepresent disclosure. That is, the unevenness of the present disclosuremay be arranged on the surface of any optical sheet constituting thebacklight unit 40. It is, however, effective to arrange the unevennessof the present disclosure to the surface of the diffusion sheet arrangedclosest to (directly above) the small light sources 42, to improve thereflection characteristics.

A method of manufacturing the optical sheet 43 is not limited. Forexample, extrusion molding or injection molding may be employed.Single-layer diffusion sheets with uneven surfaces may be manufacturedby extrusion molding as follows. First, plastic particles as pelletsadded with a diffusing agent are introduced into a single-screwextruder. In addition, those plastic particles which are not added withany diffusing agent may also be mixed. The plastic particles are moltenand kneaded while being heated. A molten resin extruded through T-diesis then sandwiched between two metal rolls and cooled. After that, theresin is transported by guide rolls to be cut off into sheet plates by asheet cutter machine, resulting in fabrication of the diffusion sheet.Here, the molten resin is sandwiched between the metal rolls, one ofwhich has a surface with predetermined inverted unevenness, which willbe transferred onto the resin. This allows for shaping of diffusionsheets to have surfaces with the desired unevenness. However, thesurface shapes of the rolls are not 100% transferred onto the resin andmay thus be counted backward from the degree of transfer to be designed.

If a double-layer diffusion sheet with uneven surfaces is manufacturedby extrusion molding, for example, plastic particles as pelletsnecessary for forming each layer are introduced into one of twosingle-screw extruders. The procedure as above is then performed foreach layer. The fabricated sheets are layered.

Alternatively, plastic particles as pellets necessary for forming eachlayer are introduced into one of two single-screw extruders. The plasticparticles are molten and kneaded while being heated. After that, moltenresins to be the layers are introduced into a single T-die to be layeredtherein. The multilayer of the molten resins extruded through the T-dieis then sandwiched between two metal rolls and cooled. After that, themultilayer is transported by guide rolls to be cut off into sheet platesusing a sheet cutter machine, resulting in fabrication of a double-layerdiffusion sheet with an uneven surface.

In this embodiment, the backlight unit 40 is a direct backlight unit inwhich a plurality of small light sources 42 are distributed at a side ofthe liquid crystal display device 50 opposite to a display screen 50 a.A decrease in the distance between the small light sources 42 and theoptical sheet 43 is needed to miniaturize the liquid crystal displaydevice 50. However, a decrease in this distance tends to cause thephenomenon (i.e., non-uniformity of luminance) that the regions of thedisplay screen 50 a above the spaces between the distributed small lightsources 42 have a lower luminance than the other regions.

By contrast, the optical sheet of the present disclosure with unevensurfaces as described above is useful to reduce such non-uniformity ofluminance. In particular, the present disclosure is believed to be moreuseful if the distance between the small light sources and the opticalsheet is set 15 mm or less, preferably 10 mm or less, more preferably 5mm or less, further more preferably 2 mm or less, and ultimately 0 mm,aiming to reduce the thickness of small to medium size liquid crystaldisplay in the future.

Examples and Comparative Example

The following describes an optical sheet of Examples and ComparativeExamples.

The steps 1 to 6 described above were used to calculate the inclinationangles of the uneven surfaces of the optical sheet in the examples, andto calculate the “ratio of the area with inclination angles of 30° ormore” of the optical sheet in the examples.

The luminance uniformity was evaluated as follows in each example toprovide universal evaluation considering the influence of thelight-emitting characteristics of LEDs used as light sources and thearrangement of LEDs. First, after one LED had been turned on, theluminance of the LED image on the two-dimensional plane was measuredfrom the upper side of a polarizer sheet (e.g., DBEF series manufacturedby 3M) to obtain variations in the luminance according to the distancefrom the center of the LED. At that time, the optical sheet to beevaluated was disposed at a certain distance from the LED, two prismsheets were layered on the optical sheet with their ridge linesorthogonal to each other, and the polarizer sheet was disposed on theprism sheets. FIG. 9 illustrates a two-dimensional luminancedistribution of an LED image obtained by the two-dimensional luminancemeasurement described above using RISACOLOR manufactured by HI-LAND.Next, a luminance variation curve was extracted along a straight line(i.e., a white line extending along the x-axis in FIG. 9) passingthrough the central point of the LED to form a graph with the horizontalaxis representing the distance from the center of the LED and thevertical axis representing a relative luminance to the maximum luminanceof 1, as shown in FIG. 10. The width of the graph along the horizontalaxis was obtained at the relative luminance of 0.5, as the full width athalf maximum (FWHM). The width of the graph along the horizontal axiswas obtained at the relative luminance of 0.1, as the FW0.1M. A largerFWHM means a wide spread of light and improved luminance uniformity. TheFWHM was thus used to evaluate the luminance uniformity in each of theexamples.

In each example, the base resin (i.e., the matrix) of the optical sheetis made of polycarbonate with a refractive index of 1.59, whereas thediffusing agent added to the base resin is made of silicone with arefractive index 1.43. That is, the difference in refractive indexbetween the base resin and the diffusing agent is 0.16 in the opticalsheet of each example.

Comparative Example 1

An optical sheet according to Comparative Example 1 was fabricated bythe following method. First, a resin containing 5% by mass of adiffusing agent with respect to 100% by mass of a base resin wassubjected to extrusion molding to form a film. Then, as one of tworolls, a roll having a random matte surface with Ra (arithmetic meanroughness) of 4.5 μm was pressed onto the resin film at a linearpressure of 30 kg/cm to transfer its surface shape to the resin film,thereby fabricating a single-layer optical sheet (thickness: 1 mm)having unevenness on its surface.

The surface shape of the optical sheet thus fabricated according toComparative Example 1 is shown in FIG. 11. Further, values of the “Ra”,the “ratio of the area with inclination angles of 30° or more”, and the“FWHM” of the optical sheet of Comparative Example 1 are shown in FIG.18. As shown in FIG. 18, in Comparative Example 1, the “Ra” is 3.632 μm,the “ratio of the area with inclination angles of 30° or more” is 25%,and the “FWHM” is 10.97434 mm.

Example 1

An optical sheet of Example 1 was fabricated by the following method.First, a resin containing 5% by mass of a diffusing agent with respectto 100% by mass of a base resin was subjected to extrusion molding toform a film. Then, as one of two rolls, a roll having a random mattesurface with Ra of 14.0 μm was pressed onto the resin film at a linearpressure of 30 kg/cm to transfer its surface shape to the resin film,thereby fabricating a single-layer optical sheet (thickness: 1 mm)having unevenness on its surface.

The surface shape of the optical sheet thus fabricated according toExample 1 is shown in FIG. 12. Further, values of the “Ra”, the “ratioof the area with inclination angles of 30° or more”, and the “FWHM” ofthe optical sheet of Example 1 are shown in FIG. 18. As shown in FIG.18, in Example 1, the “Ra” is 11.427 μm, the “ratio of the area withinclination angles of 30° or more” is 36%, and the “FWHM” is 11.13328mm.

Comparative Example 2

An optical sheet of Comparative Example 2 was fabricated by thefollowing method. First, a resin containing 5% by mass of a diffusingagent with respect to 100% by mass of a base resin was subjected toextrusion molding to form a film. Then, as one of two rolls, a rollhaving pyramid shapes on its surface was pressed onto the resin film ata linear pressure of 5 kg/cm to transfer its surface shape to the resinfilm, thereby fabricating a single-layer optical sheet (thickness: 1 mm)having inverted pyramid shapes on its surface.

The surface shape of the optical sheet thus fabricated according toComparative Example 2 is shown in FIG. 13. Further, values of the “Ra”,the “ratio of the area with inclination angles of 30° or more”, and the“FWHM” of the optical sheet of Comparative Example 2 are shown in FIG.18. As shown in FIG. 18, in Comparative Example 2, the “Ra” is 0.443 μm,the “ratio of the area with inclination angles of 30° or more” is 17%,and the “FWHM” is 10.4808 mm.

Example 2

An optical sheet of Example 2 was fabricated by the following method.First, a resin containing 5% by mass of a diffusing agent with respectto 100% by mass of a base resin was subjected to extrusion molding toform a film. Then, as one of two rolls, a roll having pyramid shapes onits surface was pressed onto the resin film at a linear pressure of 10kg/cm to transfer its surface shape to the resin film, therebyfabricating a single-layer optical sheet (thickness: 1 mm) havinginverted pyramid shapes on its surface.

The surface shape of the optical sheet thus fabricated according toExample 2 is shown in FIG. 14. Further, values of the “Ra”, the “ratioof the area with inclination angles of 30° or more”, and the “FWHM” ofthe optical sheet of Example 2 are shown in FIG. 18. As shown in FIG.18, in Example 2, the “Ra” is 0.528 μm, the “ratio of the area withinclination angles of 30° or more” is 30%, and the “FWHM” is 11.145 mm.

Example 3

An optical sheet of Example 3 was fabricated by the following method.First, a resin containing 5% by mass of a diffusing agent with respectto 100% by mass of a base resin was subjected to extrusion molding toform a film. Then, as one of two rolls, a roll having pyramid shapes onits surface was pressed onto the resin film at a linear pressure of 20kg/cm to transfer its surface shape to the resin film, therebyfabricating a single-layer optical sheet (thickness: 1 mm) havinginverted pyramid shapes on its surface.

The surface shape of the optical sheet thus fabricated according toExample 3 is shown in FIG. 15. Further, values of the “Ra”, the “ratioof the area with inclination angles of 30° or more”, and the “FWHM” ofthe optical sheet of Example 3 are shown in FIG. 18. As shown in FIG.18, in Example 3, the “Ra” is 1.070 μm, the “ratio of the area withinclination angles of 30° or more” is 52%, and the “FWHM” is 11.201 mm.

Example 4

An optical sheet of Example 4 was fabricated by the following method.First, a resin containing 5% by mass of a diffusing agent with respectto 100% by mass of a base resin was subjected to extrusion molding toform a film. Then, as one of two rolls, a roll having pyramid shapes onits surface was pressed onto the resin film at a linear pressure of 35kg/cm to transfer its surface shape to the resin film, therebyfabricating a single-layer optical sheet (thickness: 1 mm) havinginverted pyramid shapes on its surface.

The surface shape of the optical sheet thus fabricated according toExample 4 is shown in FIG. 16. Further, values of the “Ra”, the “ratioof the area with inclination angles of 30° or more”, and the “FWHM” ofthe optical sheet of Example 4 are shown in FIG. 18. As shown in FIG.18, in Example 4, the “Ra” is 2.435 μm, the “ratio of the area withinclination angles of 30° or more” is 90%, and the “FWHM” is 11.6638 mm.

Example 5

An optical sheet of Example 5 was fabricated by the following method.First, a first uniaxial extruder was used to form a molten resincontaining 5% by mass of a diffusing agent with respect to 100% by massof the base resin, and a second uniaxial extruder was used to form amolten resin containing no diffusing agent. Next, both molten resinswere supplied to a single T-die and layered within the T-die. To alayered molten resin extruded from the T-die, a roll having pyramidshapes on its surface was used as one of two metal rolls and pressedonto the resin film at a linear pressure of 10 kg/cm to transfer itssurface shape to a side of the molten resin containing no diffusingagent. In this way, a two-layer optical sheet with a total thickness of1 mm, having inverted pyramid shapes on its surface, was fabricated. Theupper layer having the inverted pyramid shapes was 0.1 mm in thickness,whereas the lower layer with a flat surface was 0.9 mm in thickness.

The surface shape of the optical sheet thus fabricated according toExample 5 is shown in FIG. 17. Further, values of the “Ra”, the “ratioof the area with inclination angles of 30° or more”, and the “FWHM” ofthe optical sheet of Example 5 are shown in FIG. 18. As shown in FIG.18, in Example 5, the “Ra” is 0.553 μm, the “ratio of the area withinclination angles of 30° or more” is 32%, and the “FWHM” is 11.6034 mm.

FIG. 19 illustrates a correlation between the “ratio of the area withinclination angles of 30° or more” and the “FWHM” of optical sheets ofExamples 1 to 5 and those of Comparative Examples 1 and 2.

As shown in FIG. 19, the value of FWHM that indicates the luminanceuniformity shows a tendency of rapidly decreasing when the “ratio of thearea with inclination angles of 30° or more” drops below 30%. Theresults of Examples 1 to 4 show that, in cases of a single layer opticalsheet, the FWHM tends to increase with an increase in the “ratio of thearea with inclination angles of 30° or more”. Further, from the resultof Example 5, it should be understood that the layer with unevenness onits surface preferably contains no diffusing agent for the purpose ofimproving the reflection characteristics.

FIG. 20 illustrates a relation between the “roughness (Ra)” and the“FWHM” of optical sheets of Examples 1 to 5 and those of ComparativeExamples 1 and 2.

As should be understood from FIG. 20, no correlation was found betweenthe “roughness” and the “FWHM”. This is because, the “ratio of the areawith inclination angles of 30° or more” as well as the reflectioncharacteristics are basically the same if the shapes of the unevennessare similar, whereas the value of the roughness (surface roughness)varies depending on the sizes of the shapes of the unevenness, even ifthe shapes of the unevenness are similar. Thus, the “ratio of the areawith inclination angles of 30° or more” can be considered as a suitableindex, in evaluation of the reflection characteristics and the luminanceuniformity of an optical sheet.

Other Embodiments

Although the optical sheet 43 shown in FIG. 3 has a diffusion layer 21and an uneven layer 22 formed on the diffusion layer 21, it is possibleto structure the optical sheet 43 with only one uneven layer 22 as shownin FIG. 21. In this case, the surface of the optical sheet 43 having norecesses 22 a may be a mirror surface or a matte surface. It should benoted that, in FIG. 21, the same reference characters are given tostructuring elements that are identical to those of the optical sheet 43shown in FIG. 3.

The diffusing agent contained in the optical sheet 43 (uneven layer 22)shown in FIG. 21 may be 0 part by weight or more and 4 parts by weightor less, more preferably 0 part by weight or more and 2 parts by weightor less, and further preferably 0 part by weight or more and 1 part byweight or less, with respect to 100 parts by weight of a matrix resin.

The matrix resin is preferably an aromatic polycarbonate resin. Forexample, the matrix resin may be obtained through a reaction of adihydric phenol and a carbonate precursor by an interfacialpolymerization method or a fusion method.

Examples of typical dihydric phenol include:2,2-bis(4-hydroxyphenyl)propane (commonly known as bisphenol A),1,1-bis(4-hydroxyphenyl)ethane, 1,1-bis(4-hydroxyphenyl)cyclohexane,2,2-bis(3-methyl-4-hydroxyphenyl)propane,2,2-bis(3,5-dimethyl-4-hydroxyphenyl)propane,bis(4-hydroxyphenyl)sulfide, bis(4-hydroxyphenyl)sulfone, and the like.Of these, bisphenol A is preferable. These dihydric phenols may be usedalone or as a mixture of two or more kinds.

Examples of carbonate precursors include a carbonyl halide, a carbonateester, a haloformate, and the like. Specific examples of the carbonateprecursor include phosgene, diphenyl carbonate, and dihaloformate of adihydric phenol, and the like.

A molecular weight modifier, a catalyst, and the like may be used asneeded, at a time of producing the polycarbonate resin through areaction of the dihydric phenol and the carbonate precursor by aninterfacial polymerization method or a fusion method. To thepolycarbonate resin, an additive may be blended as needed. Examples ofthe additive include a parting agent such as an ester or partial esterof a polyhydric alcohol and a fatty acid, a heat stabilizer such as aphosphite ester, phosphoric acid ester, or phosphonic acid ester, anultraviolet absorber such as a benzotriazole-based ultraviolet absorber,an acetophenone ultraviolet absorber, or a salicylic acid ester, anantistatic agent, a coloring agent, a whitener, or a flame retardant.The polycarbonate resin may be a branched polycarbonate resin obtainedby copolymerizing a polyfunctional aromatic compound having three ormore functional groups. The polycarbonate resin may be a polyestercarbonate resin obtained by copolymerizing an aromatic or aliphaticbifunctional carboxylic acid. The polycarbonate resin may be a mixtureof two or more kinds of polycarbonate resins obtained.

The molecular weight of the polycarbonate resin is preferably 10,000 to100,000, more preferably 15,000 to 35,000, in the viscosity-averagemolecular weight (M). The polycarbonate resin having such aviscosity-average molecular weight is preferable, as it is sufficientlystrong and exhibits a good melt flowability during molding. The“viscosity-average molecular weight” herein is obtained by applying aspecific viscosity (ηsp) derived from a solution obtained in which 0.7 gof polycarbonate resin is dissolved in 100 ml of methylene chloride at20° C.ηsp/C=[η]+0.45×[η]² C,

where [η] is a limiting viscosity and is 1.23×10⁻⁴M^(0.83)C=0.7.

The diffusing agent is preferably a silicone-based diffusing agent. Forexample, the diffusing agent may be spherical microparticles having asiloxane bond, whose mean particle size is 0.8 μm or more and 12 μm orless. With the mean particle size of the diffusing agent falling shortof 0.8 μm, it tends to be difficult to achieve sufficient lightdiffusibility even if the amount of diffusing agent added is varied. Onthe other hand, it tends to be difficult to achieve a favorable lightdiffusibility with the mean particle size exceeding 12 μm, even if theamount of diffusing agent added is varied. Examples of typicallyadoptable silicone-based diffusing microparticles include silica,silicone resin, silicone rubber, silicone complex powder which isspherical powder obtained by coating the surfaces of spherical siliconerubber powder with silicone resin, and a combination of these. Of these,silicone complex powder is particularly preferable.

Other Examples

Optical sheets of Examples 6 to 20 were fabricated by the followingmethod. First, to 100 parts by weight of an aromatic polycarbonateresin, a silicone complex powder (mean particle size: 2.0 μm) as thediffusing agent was added so that the composition was as shown in FIG.22, then the mixture was molten and fused in an extruder and extrudedfrom a T-die. Then, as one of two metal rolls, a roll having on itssurface square pyramid shapes of 50 μm in height and 90° in vertex angleat a pitch of 100 μm was used. As the other roll, a roll having a randommatte surface (surface roughness Ra=4 μm) was used for Example 6 to 18,and a roll with a mirror surface (surface roughness Ra=0.01 μm) was usedfor Examples 19 and 20. Sandwiching the molten resin extruded from theT-die between these two rolls, shapes on the rolls were transferredwhile the molten resin was cooled. In this way, single layer opticalsheets with the thicknesses as shown in FIG. 22 were fabricated. Each ofthe optical sheets has (inverted) pyramid shapes of 45 μm in depth onone surface, and has, on the other side, a matte surface or a mirrorsurface with the surface roughness shown in FIG. 22.

FIG. 22 illustrates, for each of the optical sheets of Examples 6 to 20,the “surface shapes (including ratio of the area with inclination anglesof 30° or more) and surface roughness”, the “FWHM”, the “FW0.1M”, andthe “average values of transmittance and reflectance for spectral beamsof 400 nm to 700 nm in wavelength”. The values of the “FWHM”, the “FW0.1M”, and the “average values of transmittance and reflectance forspectral beams of 400 nm to 700 nm in wavelength” were measured with thematte surface or mirror surface of the optical sheet as the lightincident side, and the surface with the inverted pyramids as the lightemitting side.

As shown in FIG. 22, with the optical sheets of Examples 6 to 20, the“FWHM” and the “FW0.1 M” increases thus improving the diffusibility,with a decrease in the content of the diffusing agent. Specifically, thecontent of the diffusing agent is preferably 0 part by weight or moreand 4 parts by weight or less, more preferably 0 part by weight or moreand 2 parts by weight or less, and further preferably 0 part by weightor more and 1 part by weight or less, with respect to 100 parts byweight of a matrix resin.

The optical sheet of Example 21 includes three optical sheets fabricatedin Example 6 overlapped with one another. The optical sheet of Example22 includes three optical sheets fabricated in Example 7 overlapped withone another. The optical sheet of Example 23 includes three opticalsheets fabricated in Example 8 overlapped with one another. The opticalsheet of Example 24 includes three optical sheets fabricated in Example9 overlapped with one another. The optical sheet of Example 25 includesthree optical sheets fabricated in Example 11 overlapped with oneanother. The optical sheet of Example 26 includes three optical sheetsfabricated in Example 12 overlapped with one another. The optical sheetof Example 27 includes three optical sheets fabricated in Example 13overlapped with one another. The optical sheet of Example 28 includesthree optical sheets fabricated in Example 14 overlapped with oneanother. The optical sheet of Example 29 includes four films overlappedwith one another. Each of the films has the same composition as theoptical sheet fabricated in Example 6 and has a thickness of 115 μm. Theoptical sheet of Example 30 includes four films overlapped with oneanother. Each of the films has the same composition as the optical sheetfabricated in Example 9 and has a thickness of 115 μm. The optical sheetof Example 31 includes four films overlapped with one another. Each ofthe films has the same composition as the optical sheet fabricated inExample 11 and has a thickness of 115 μm. The optical sheet of Example32 includes four films overlapped with one another. Each of the filmshas the same composition as the optical sheet fabricated in Example 13and has a thickness of 115 μm.

FIG. 23 illustrates a cross-sectional structure of the backlight unit ata time of evaluating the luminance uniformity for the optical sheets ofExamples 21 to 32. FIG. 24 shows an arrangement of light sources in thebacklight unit 40 shown in FIG. 23. It should be noted that, in FIG. 23,the same reference characters are given to structuring elements that areidentical to those of the backlight unit 40 shown in FIG. 2. For thesake of easier understanding, the optical sheet 43 of FIG. 23 has onlyone layer.

In the measurement of the “luminance uniformity”, a predetermined numberof optical sheets 43 each having inverted pyramid shapes are arrangedabove arrays of small light sources 42 (LED array), and two prism sheets44 and 45 are arranged above the optical sheets 43, as shown in FIG. 23and FIG. 24. The distance from the LED array to the upper most surfaceof the prism sheet was unified to 2 mm. As the LED array, an array withLEDs arranged at a pitch of 7 mm was used. LEDs (small light sources 42)used were blue LEDs of product number XPGDRY-L1-0000-00501 manufacturedby Cree.

The “luminance uniformity” was evaluated as follows. First, using an LEDarray (six times six) as shown in FIG. 24, cross-sectional luminanceswere obtained along a diagonal line passing through immediately abovethe LEDs (small light sources 42). Then, the average and the standarddeviation of the cross-sectional luminances were calculated. The“luminance uniformity” was derived by a formula of the “luminanceuniformity (%)”=(average value−standard deviation)/(averagevalue+standard deviation)×100. The higher the value of the “luminanceuniformity” derived is, the more uniform the luminance is.

FIG. 25 illustrates, for each of the optical sheets of Examples 21 to32, the “luminance uniformity”, the “average transmittance andreflectance for spectral beams of 400 nm to 700 nm in wavelength”, andthe “ratio of the area with inclination angles of 30° or more”. Thevalues of the “luminance uniformity” and the “average transmittance andreflectance for spectral beams of 400 nm to 700 nm in wavelength” weremeasured with the matte surface of the optical sheet as the lightincident side, and the surface with the inverted pyramids as the lightemitting side. The values of the “ratio of the area with inclinationangles of 30° or more” were calculated as follows. Namely, the “ratio ofthe area with inclination angles of 30° or more” was measured for eachof the three overlapped films in the optical sheets of Examples 21 to28, and for each of four overlapped sheets in the optical sheets ofExamples 29 to 32. Then, the “ratio of the area with inclination anglesof 30° or more” was derived as the average value of the measured values.

From the optical sheets of Examples 21 to 32 as shown in FIG. 25, itshould be understood that luminance uniformity increases with a decreasein the content of diffusing agent, when a plurality of films each havingthe same structure (thickness, composition, and the like) are used andoverlapped with one another.

In view of the foregoing description, the embodiments (including theExamples. The same applies for the following description.) of thepresent disclosure are described. However, the present disclosure is notlimited to the above-described embodiments alone, and variousmodifications may be made within the scope of the present disclosure.That is, the description of the foregoing embodiments is merelyexemplary in nature, and is not intended to limit the application or theuse of the present disclosure.

For example, as shown in a modification of FIG. 26, printed portions 23,e.g., dots of white ink, that block and/or reflect light from the smalllight sources 42 may be formed on a portion of one surface (the uppersurface of the uneven layer 22) closer to the display screen 50 a, ofthe optical sheet 43. The above portion of the optical sheet 43 isopposite to another portion (the portion of the lower surface of thediffusion layer 21 shown by R in the figure) of another surface closerto the small light sources 42, of the optical sheet 43. The otherportion faces the small light sources 42. In FIG. 26, the areas wherethe printed portions 23 are arranged are shown in white, andillustration of recesses 22 a are omitted.

The printed portions 23 of the present modification further improve theluminance uniformity, as they bring about reflection effect of lightfrom the small light sources 42 and/or an effect of blocking excessivelight from the small light sources 42 in a direction towards directlyabove.

In the present modification, the printed portions 23 may be formed on atleast one of the surfaces of the optical sheet 43. Further, in cases offorming the printed portions 23 on one surface of an optical sheet 43having unevenness on one of its surfaces only, the printed portions 23may be formed on the surface having the unevenness or on the surface ofthe other side.

The invention claimed is:
 1. An optical sheet to be interposed between aplurality of small light sources and a prism sheet in a liquid crystaldisplay device having the small light sources arranged in a dispersedmanner at a side of the liquid crystal display device opposite to adisplay screen, wherein: the optical sheet includes a plurality ofoptical sheets overlapped relative to one another, each of the pluralityof optical sheets comprises 0 part by weight or more and 2 parts byweight or less of a diffusing agent with respect to 100 parts by weightof a matrix resin constituting each of the plurality of optical sheets,each of the plurality of optical sheets has one surface having recesses,each recess having an inverted pyramid shape and being two-dimensionallyarrayed, 30% or more regions of a plurality of minute regions areinclined at inclination angles of 30° or more from a flat surface, theplurality of minute regions are two-dimensionally arrayed at equalintervals on the one surface, each of the plurality of minute regionshas an area of 100 μm² or less; and a distance between the plurality ofsmall light sources and the optical sheet is 2 mm or less.
 2. Theoptical sheet of claim 1, wherein: printed portions that block and/orreflect light from the small light sources are formed on a first portionof one surface of the optical sheet closer to the small light sourcesand/or on a second portion of another surface of the optical sheetcloser to the display screen, and the first portion faces the smalllight sources and the second portion is opposite to the first portion.3. The optical sheet of claim 1, wherein: the matrix resin is anaromatic polycarbonate resin, and the diffusing agent is asilicone-based diffusing agent.
 4. The optical sheet of claim 1, whereineach of the optical sheets comprises 0 part by weight or more and 1parts by weight or less of the diffusing agent with respect to 100 partsby weight of the matrix resin.
 5. The optical sheet of claim 1, whereineach of the optical sheets does not contain the diffusing agent,substantially.
 6. A method for evaluating the optical sheet of claim 1,the method comprising: when at least one scope of 0.5 mm square or moreof the surface of the optical sheet having the unevenness is measured,determining height data of each of a plurality of pixels of an imageobtained, calculating an approximated surface, from the height data ofeach pixel, for a minute region of 100 μm² or less including a pluralityof pixels, repeatedly performing a calculation to obtain an inclinationangle between: (i) a flat surface (with a height of 0) appearing afterthe unevenness is imaginarily removed and: (ii) the approximatedsurface, while two-dimensionally shifting the minute region at equalintervals along the flat surface by using at least one of the pixels asa unit to obtain a plurality of minute regions, and calculating a ratioof a total area of some of the minute regions each having theinclination angle of 30° or more with respect to a total area of all theminute regions, for which the calculation is made repeatedly.
 7. Abacklight unit to be built into a liquid crystal display device andconfigured to guide light emitted from a plurality of small lightsources towards a display screen, the unit comprising: an optical sheetpositioned between the display screen and the plurality of small lightsources, wherein: the optical sheet includes a plurality of opticalsheets overlapped relative to one another, each of the plurality ofoptical sheets comprises 0 part by weight or more and 2 parts by weightor less of a diffusing agent with respect to 100 parts by weight of amatrix resin constituting each of the plurality of optical sheets, eachof the plurality of optical sheets has one surface having recesses, eachrecess having an inverted pyramid shape and being two-dimensionallyarrayed, 30% or more regions of a plurality of minute regions areinclined at inclination angles of 30° or more from a flat surface, theplurality of minute regions are two-dimensionally arrayed at equalintervals on the one surface, each of the plurality of minute regionshas an area of 100 μm² or less; and a distance between the plurality ofsmall light sources and the optical sheet is 2 mm or less.
 8. Thebacklight unit of claim 7, wherein the plurality of small light sourcesare each an LED element.
 9. The backlight unit of claim 7, wherein theplurality of small light sources are regularly arranged.
 10. Thebacklight unit of claim 7, wherein the plurality of small light sourcesare arranged on a reflection sheet that is opposite to the optical sheetacross the small light sources.
 11. A liquid crystal display devicecomprising: the backlight unit of claim 7, and a liquid crystal displaypanel.
 12. An information device, comprising the liquid crystal displaydevice of claim
 11. 13. The backlight unit of claim 7, wherein thebacklight unit is a local-dimming back light unit.
 14. The backlightunit of claim 7, wherein, among the plurality of optical sheets, anaverage value of a ratio of the regions, which are inclined at theinclination angles of 30° or more, to the plurality of minute regions is80% or more.
 15. The backlight unit of claim 7, wherein the distancebetween the plurality of small light sources and the optical sheet is 1mm or less.